A conventional laundry treatment apparatus includes a cabinet defining an appearance of the apparatus, a tub installed in the cabinet, a drum rotatably installed in the tub to wash laundry, and a motor having a rotating shaft, coupled to the drum while passing through the tub, to rotate the drum.
The drum may rotate without maintaining dynamic equilibrium (dynamic balance) depending on a position of laundry disposed therein.
“Dynamic equilibrium” means a state that, during rotation of a rotating body, a centrifugal force of the rotating body or a moment caused by the centrifugal force becomes zero with respect to the axis of rotation. In case of a rigid body, dynamic equilibrium is maintained when mass of the rigid body is evenly distributed about the axis of rotation.
Accordingly, dynamic equilibrium of a laundry treatment apparatus may be considered as a state in which the mass distribution of laundry about an axis of rotation of a drum including laundry contained therein falls within an allowable range during rotation of the drum (a state in which the drum rotates in an allowable amplitude range of vibration).
Meanwhile, a state wherein the dynamic equilibrium in a laundry treatment apparatus is lost (a state of unbalance) means that mass distribution of laundry about the axis of rotation of a drum is non-uniform during rotation of the drum. The loss of dynamic equilibrium occurs when laundry is not evenly distributed along an inner surface of the drum.
When a drum rotates in an unbalanced state, vibration is generated. The vibration of the drum is transmitted to a tub or a cabinet thus causing generation of noise.
Among conventional laundry treatment apparatuses, there is an apparatus equipped with a balancer for improving an unbalanced state of a drum. A balancer incorporated in conventional laundry treatment apparatuses is a ball balancer or a fluid balancer in which a ball or fluid is contained in a housing fixed to a drum.
When a drum is in the unbalanced state, the drum exhibits the highest rotational speed when laundry incurring the unbalanced state passes through the lowest point of the rotational orbit of the drum and exhibits the lowest rotational speed when the laundry incurring the unbalanced state passes through the highest point of the rotational orbit of the drum.
Therefore, the ball balancer or the fluid balancer incorporated in conventional laundry treatment apparatuses controls unbalanced state in such a manner that a ball or fluid moves toward the lowest point of a rotational orbit of a drum when laundry incurring the unbalanced state moves toward the highest point.
Although the above-mentioned unbalance control is useful under the vibration condition in a steady state in which amplitude of vibration of a drum falls within a predetermined range, satisfactory effects cannot be obtained under transient vibration to which the drum is subjected before vibration of the drum reaches the steady state.
Furthermore, conventional balancing units have difficulty in immediately resolving (actively resolving) unbalanced state upon occurrence of unbalance.